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Full text of "Mathematical And Physical Papers - Iii"

316          ON THE  CHANGE  OF  REFEANGIBILITY OF  LIGHT.
which were examined by the third and fourth methods were found to exhibit a little false dispersion, produced chiefly in the brightest part of the spectrum, but the greater part was true dispersion. This dispersion was produced chiefly by a rather narrow band, comprising the fixed line 6r, where there appeared to be a remarkable maximum of sensibility. The line 0 lay a little above the lower limit of the band. Below the band dispersion also took place, though not near so copiously, and there appeared to be another maximum of sensibility some way further down in the spectrum; but above the band dispersion almost entirely ceased of a sudden; a very unusual circumstance when the active and the dispersed light are well separated in re-frangibility. The position of the band in the spectrum, and the distribution of the illumination in it, which are very peculiar, were the same in all the specimens which were sufficiently sensitive to admit of being examined by the third method, but the tint of the dispersed light was not quite the same.
79.    Orange-coloured glasses are frequently met with which reflect from one side, or rather scatter in all directions, a copious light of a bluish-green colour, quite different from the transmitted : |J                               tint.    In such cases the body of the glass is colourless, and the
 fj                               colouring matter is contained in a very thin layer on one face of
| H                               the plate.    The bluish-green tint is seen when the colourless face
; '|lj                              is next the eye.    As this phenomenon was supposed by Sir John
'{!                               Herschel to offer some analogy with the reflected tints of fluor-
<*,'                              spar and a solution of sulphate of quinine, I was the more desirous
of determining the nature of the dispersion.    It proved on exam-ji                              ination to be nothing but false dispersion, so that the appearance
''                              might be conceived to be produced by an excessively fine bluish-
ji                              green  powder  contained  in  a  clear  orange  stratum, or in the
1                               colourless part of the glass immediately contiguous to the coloured
!j                           stratum.    The phenomenon has therefore no relation to the tints
1                           of fluor-spar or sulphate of quinine.    It is true that the very
j                           same glass which displayed a superficial reflexion of bluish green,
J                           when   examined  by condensed  sunlight   exhibited  also,  in  its
,-j                           colourless part, a little true dispersion, just as another colourless
,!'                           glass would  do.    But  this has plainly nothing to do with the
|i                           peculiar reflexion which attracts notice in such a glass.